A technique for assisting driver's driving by detecting an obstacle around an own vehicle with a sensor is known. For example, Patent Document 1 discloses the technique for detecting a side-by-side running vehicle in a blind spot and a relative velocity of the side-by-side running vehicle with an ultrasonic sensor. In the present disclosure, the side-by-side running vehicle refers to a vehicle that is running side-by-side with the own vehicle.
More specifically, according to the technique of Patent Document 1, frequency analysis is performed on a transmitted wave and a received ultrasonic wave to detect a frequency difference between the transmitted wave and the received wave. Based on the frequency difference, the relative velocity of the side-by-side running vehicle is detected. That is, the technique uses a Doppler effect, which shifts the frequency when the ultrasonic wave is incident on a moving body at a certain angle and is returned from the moving body. Based on a frequency shift amount, the relative velocity is detected.
Furthermore, according to the technique of Patent Document 1, a frequency band of a receiver of the ultrasonic sensor is limited to “f±α (kHz)”, where f is the frequency of the transmitted wave. When the frequency shift amount of the reflected wave detected with the receiver is less than “α”, it is determined that an object having the small relatively velocity is detected, i.e., the side-by-side running vehicle is detected.    Patent Document 1: JP-H8-268189A
According to the technique of Patent Document 1, since the frequency band of the receiver of the ultrasonic sensor is limited to “f±α (kHz) in order to detect the side-by-side running vehicle, the reflected wave having the frequency out of the limited band is not received. Therefore, objects other than side-by-side running vehicle cannot be detected. System versatility is disadvantageously low.
Furthermore, according to the technique of Patent Document 1, the frequency analysis is performed to detect the frequency difference between the transmitted wave and the received wave. However, the frequency analysis such as FFT (Fast Fourier Transform) typically requires a complicated analysis to detect the frequency. Therefore, a large amount of time and work is disadvantageously required.